Spacer chalcogenide memory method and device

ABSTRACT

The present invention includes devices and methods to form memory cell devices including a spacer comprising a programmable resistive material alloy. Particular aspects of the present invention are described in the claims, specification and drawings.

BACKGROUND OF THE INVENTION

Chalcogenide materials are widely used in read-write optical disks.These materials have at least two solid phases, generally amorphous andgenerally crystalline. Laser pulses are used in read-write optical disksto switch between phases and to read the optical properties of thematerial after the phase change.

Chalcogenide materials also can be caused to change phase by applicationof electrical current. This property has generated interest in usingprogrammable resistive material to form nonvolatile memory circuits.

One direction of development has been toward using small quantities ofprogrammable resistive material, particularly in small pores. Patentsillustrating development toward small pores include: Ovshinsky,“Multibit Single Cell Memory Element Having Tapered Contact,” U.S. Pat.No. 5,687,112, issued Nov. 11, 1997; Zahorik et al., “Method of MakingChalogenide [sic] Memory Device,” U.S. Pat. No. 5,789,277, issued Aug.4, 1998; Doan et al., “Controllable Ovonic Phase-Change SemiconductorMemory Device and Methods of Fabricating the Same,” U.S. Pat. No.6,150,253, issued Nov. 21, 2000.

Accordingly, an opportunity arises to devise methods and structures thatform memory cells with structures that use small quantities ofprogrammable resistive material.

SUMMARY OF THE INVENTION

The present invention includes devices and methods to form memory celldevices including a spacer comprising a programmable resistive materialalloy. Particular aspects of the present invention are described in theclaims, specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of a multilayer structure.

FIG. 1B is a block diagram of a multilayer structure with a depositedchalcogenide alloy layer.

FIG. 1C is a block diagram of a multilayer structure with chalcogenidespacers.

FIG. 2 is a block diagram of an alternate multilayer structure withchalcogenide spacers.

FIG. 3 is a block diagram of a multilayer structure with chalcogenidespacers and an additional isolation transistor.

DETAILED DESCRIPTION

The following detailed description is made with reference to thefigures. Preferred embodiments are described to illustrate the presentinvention, not to limit its scope, which is defined by the claims. Thoseof ordinary skill in the art will recognize a variety of equivalentvariations on the description that follows.

A chalcogenide alloy contains one or more elements from column six ofthe periodic table of elements. Many chalcogenide phase-change alloyshave been described in technical literature, including alloys of: Ga/Sb,In/Sb, In/Se, Sb/Te, Ge/Te, Ge/Sb/Te, In/Sb/Te, Ga/Se/Te, Sn/Sb/Te,In/Sb/Ge, Ag/In/Sb/Te, Ge/Sn/Sb/Te, Ge/Sb/Se/Te and Te/Ge/Sb/S. In thefamily of Ge/Sb/Te alloys, a wide range of alloy compositions may beworkable. The compositions can be characterized asTe_(a)Ge_(b)Sb_(100−(a+b)). One researcher has described the most usefulalloys as having an average concentration of Te in the depositedmaterials well below 70%, typically below about 60% and ranged ingeneral from as low as about 23% up to about 58% Te and most preferablyabout 48% to 58% Te. Concentrations of Ge were above about 5% and rangedfrom a low of about 8% to about 30% average in the material, remaininggenerally below 50%. Most preferably, concentrations of Ge ranged fromabout 8% to about 40%. The remainder of the principal constituentelements in this composition was Sb. These percentages are atomicpercentages that total 100% of the atoms of the constituent elements.Ovshinsky '112 patent, cols 10-11. Particular alloys evaluated byanother researcher include Ge₂Sb₂Te₅, GeSb₂Te₄ and GeSb₄Te₇. NoboruYamada, “Potential of Ge—Sb—Te Phase-Change Optical Disks forHigh-Data-Rate Recording”, SPIE v.3109, pp.28-37 (1997). More generally,a transition metal such as Cr, Fe, Ni, Nb, Pd, Pt and mixtures or alloysthereof may be combined with Ge/Sb/Te to form a phase-change alloy thathas programmable resistive properties. Specific examples of memorymaterials that may be useful are given in Ovshinsky '112 cols. 11-13,which examples are hereby incorporated by reference.

Phase-change alloys are capable being switched between a firststructural state in which the material is generally amorphous and asecond structural state in which the material is generally crystallinein its local order. These alloys are at least bistable. The termamorphous is used to refer to a relatively less ordered structure, moredisordered than a single crystal, which has the detectablecharacteristics such as high electrical resistivity. The termcrystalline is used to refer to a relatively more ordered structure,more ordered in an amorphous structure, which has detectablecharacteristics such as lower electrical resistivity than the amorphousstate. Typically, phase-change materials may be electrically switchedbetween different detectable states of local order across the spectrumbetween completely amorphous and completely crystalline states. Othermaterial characteristics affected by the change between amorphous andcrystalline phases include atomic order, free electron density andactivation energy. The material may be switched either into differentsolid phases or into mixtures of two or more solid phases, providing agray scale between completely amorphous and completely crystallinestates. The electrical properties in the material may vary accordingly.

Phase-change alloys can be changed from one phase state to another byapplication of electrical pulses. It has been observed that a shorter,higher amplitude pulse tends to change the phase-change material to agenerally amorphous state. A longer, lower amplitude pulse tends tochange the phase-change material to a generally crystalline state. Theenergy in a shorter, higher amplitude pulse is high enough to allow forbonds of the crystalline structure to be broken and short enough toprevent the atoms from realigning into a crystalline state. Appropriateprofiles for pulses can be determined, without undue experimentation,specifically adapted to a particular phase-change alloy. The physicalphase-change process has motivated research into structures that use asmall amount of programmable resistive material.

FIGS. 1C and 2 are block diagrams of novel cross-sections ofprogrammable resistive material. FIG. 1 depicts a multilayer structure,having a first electrode 103, an insulating layer 102, over the firstelectrode, in the second electrode 101, over the insulating layer. Thefirst electrode preferably is TiAlN. The first electrode preferably hasa thickness of 10 to 30 nm, which is less than the minimum lithographicfeature size of current lithographic processes. The interelectrodeinsulating layer may be silicon oxide, silicon nitride, Al₂O₃ or an ONOor SONO multi-layer structure. Alternatively, the inter-electrodeinsulating layer may comprise one or more elements selected from thegroup consisting of Si, Ti, Al, Ta, N, O, and C. The inter-electrodethickness may be 100 to 300 nm. The second electrode may be TiW. It mayhave a thickness of 200 to 400 nm. Alterntively, the electrodes may beTiN or TaN, or may comprise one or more elements selected from the groupconsisting of Ti, W, Mo, Al, Ta, Cu, Pt, Ir, La, Ni, and O. A multilayerstructure has sidewalls illustrated in FIG. 1 the right and left sidesof the multilayer structure. The multilayer structures are constructedover a non-conductive or insulating base layer 104, which may be asubstrate or a layer on top of other layers. In some embodiments, thecontact via and plug 105 may be defined through the insulating layer 104to make contact with the first electrode 103. The programmable resistivematerial in this structure forms a spacer 111″ along the sidewalls ofthe multilayer structure. The spacer structure is formed usingconventional methods of forming a spacer. Initial spacer depositionlayer thickness may be 100 to 300 nm or less. After etching, the spacewidth may be reduced to 10 to 20 nm, which is less than the minimumlithographic feature size of current lithographic processes. In thisfigure, the active region of phase change is preferably at the interfacebetween the first electrode and spacer. A low first electrode thicknessimproves device performance by reducing the interface area in whichphase change takes place. Alternatively, the current flow, electrodematerials and thicknesses could be reversed and the phase change areacould appear at the interface to the second electrode.

Useful characteristics of the programmable resistive material includethe material having at least two solid phases that can be reversiblyinduced by electrical current. These at least two phases include anamorphous phase and a crystalline phase. However, in operation, theprogrammable resistive material may not be fully converted to either anamorphous or crystalline phase. Intermediate phases or mixtures ofphases may have a detectable difference in material characteristics. Thetwo solid phases should generally be bistable and have differentelectrical properties. The programmable resistive material may be achalcogenide alloy. A chalcogenide alloy may include Ge₂Sb₂Te₅.Alternatively, it may be one of the other phase-change materialsidentified above.

FIG. 2 depicts an alternate multilayer structure, having a firstelectrode 203 formed as a buried diffusion in semiconductor base layer204. The base layer may be a substrate or a layer over other layers. Inthis multilayer structure, the insulating layer 202, is over the firstelectrode, and the second electrode 201, is over the insulating layer.The programmable resistive material 211″ forms a spacer along thesidewalls of the multilayer structure, generally corresponding to thesides of the insulating layer 202 and the second electrode 201. Theinterface between the spacer and the second electrode defines a phasechange region, as a high resistance material is more easily used for asecond electrode than for the buried diffusion. This figure illustratesa buried diffusion without need for contact via. In alternateembodiments, a contact via could connect the buried diffusion throughthe additional insulating layer to layers below.

FIG. 3 depicts an additional structure that may be combined with aspacer memory structure. In particular, an isolation transistor isillustrated. One pole 321 of the transistor is electrically connected tothe contact 105. A gate 322 controls the flow of current from the otherpole 323 to the contact. Use of an isolation transistor may be useful,as the electrical resistance of a programmable resistive material isunlikely ever to be so great as to block current leakage from the firstelectrode to the second electrode. Alternatively, an isolation junctionor an isolation diode may be incorporated in the structure.

A conventional sequence for forming a spacer is generally illustrated inFIGS. 1A-1C. FIG. 1A illustrates a multilayer structure formed usingconventional method. In FIG. 1B, a programmable resistive material 111is the deposited over the multilayer structure. Techniques fordepositing such a film include sputtering and chemical vapor deposition.A film deposited by such methods generally conforms to the structurepresented, with some filling at low spots. An anisotropic etch is usedto be moved most of the programmable resistive material, leaving spacers111″ along the sidewalls of the multilayer structure. The anisotropicetch may be a plasma etch, a reactive ion etch, or any other etchcompatible with the materials used.

While the present invention is disclosed by reference to the preferredembodiments and examples detailed above, it is understood that theseexamples are intended in an illustrative rather than in a limitingsense. It is contemplated that modifications and combinations willreadily occur to those skilled in the art, which modifications andcombinations will be within the spirit of the invention and the scope ofthe following claims.

1. A memory device, including: a multilayer structure, having a firstelectrode, an insulating layer over the first electrode and a secondelectrode over the insulating layer; wherein the multilayer structuredefines a stack having sidewalls; and a programmable resistive materialspacer along the sidewalls; wherein the programmable resistive materialhas at least two solid phases.
 2. The device of claim 1, wherein the twosolid phases are reversibly inducible by a current.
 3. The device ofclaim 2, wherein the at least two solid phases include a generallyamorphous phase and a generally crystalline phase.
 4. The device ofclaim 1, wherein the programmable resistive material is a chalcogenidealloy.
 5. The device of claim 4, wherein the chalcogenide alloy includesGe2Sb2Te5.
 6. The device of claim 4, wherein the chalcogenide alloyincludes a combination of two or more materials from the group of Ge,Sb, Te, Se, In, Ti, Ga, Bi, Sn, Cu, Pd, Pb, Ag, S, or Au.
 7. The deviceof claim 1, wherein the first electrode comprises an alloy of TiAlN andapplying a current to an interface between the first electrode and theprogrammable resistive material spacer induces phase change at theinterface.
 8. The device of claim 7, wherein the first electrode has athickness of 10 to 30 nm.
 9. The device of claim 7, wherein the firstelectrode has a thickness loss than a minimum lithographic feature sizeof a lithographic process used to form the multilayer structure.
 10. Thedevice of claim 1, wherein the first electrode has a thickness of 10 to30 nm and applying a current to an interface between the first electrodeand the programmable resistive material spacer induces phase change atthe interface.
 11. The device of claim 1, wherein the first electrodehas a thickness less than a minimum lithographic feature size of alithographic process used to form the multilayer structure and applyinga current to an interface between the first electrode and theprogrammable resistive material spacer induces phase change at theinterface.
 12. The device of claim 1, wherein the first electrodeincludes a doped region in a base layer, the base layer underlying theinsulating layer, and the sidewall is defined by the base layer, theinsulating layer and the second electrode.
 13. The device of claim 12,wherein the second electrode comprises an alloy of TiAlN and applying acurrent to an interface between the second electrode and theprogrammable resistive material spacer induces phase change at theinterface.
 14. The device of claim 13, wherein the second electrode hasa thickness of 10 to 30 nm.
 15. The device of claim 13, wherein thesecond electrode has a thickness less than a minimum lithographicfeature size of a lithographic process used to form the multilayerstructure.
 16. The device of claim 12, wherein the second electrode hasa thickness of 10 to 30 nm and applying a current to an interfacebetween the second electrode and the programmable resistive materialspacer induces phase change at the interface.
 17. The device of claim12, wherein the second electrode has a thickness less than a minimumlithographic feature size of a lithographic process used to form themultilayer structure and applying a current to an interface between thesecond electrode and the programmable resistive material spacer inducesphase change at the interface.